05000341/LER-2015-002
| Docket Number | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability |
| 3412015002R00 - NRC Website | |
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CONTINUATION SHEET
2. DOCKET
Initial Plant Conditions:
Mode 1 Reactor Power 100 percent
Description of the Event:
On March 09, 2015, the monthly Technical Specifications (TS) surveillance test, SR. 3.5.1.2, of the 480V electrical swing bus [13U] 72CF Automatic Throwover scheme was being conducted. At 1521, the alternate power supply breaker failed to close as expected after the normal power supply breaker was opened, resulting in a loss of power to Reactor Recirculation (RR) Pump [AD] discharge valves and both divisions of Residual Heat Removal (RHR) Low Pressure Coolant Injection (LPCI) [BU] Injection Valves [INV]. The 72CF throwover scheme was declared inoperable. At 1521, LCO 3.5.1, Emergency Core Cooling Systems (ECCS) - Operating, was declared not met which required entry in LCO 3.0.3. Reactor power was reduced from 100% to 81% in preparation for meeting the requirement to be in MODE 2 within. 7 hours8.101852e-5 days <br />0.00194 hours <br />1.157407e-5 weeks <br />2.6635e-6 months <br />. The loss of both divisions of RHR LPCI injection valves is reportable under 10 CFR 50.73(a)(2)(v)(D), any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident and 10 CFR 50.73(a)(2)(vii), any event where a single cause or condition caused at least one independent train or channel to become inoperable in multiple systems or two independent trains or channels to become inoperable in a single system designed to mitigate the consequences of an accident. Notifications for a 4-hour non-emergency 10 CFR 50.72(b)(2)(i) for the initiation of any nuclear plant shutdown required by the plant's Technical Specifications and an 8 hour9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> non-emergency 10 CFR 50.72(b)(3)(v) for any event or condition that at the time of discovery could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident were completed.
Troubleshooting found a permissive contact in the close circuit for the alternate power supply breaker to be in the incorrect state.
Oxidation was found to be the likely cause of the degraded condition of the contact.
Operations de-energized and tagged out the 72CF Throwover scheme to support troubleshooting and maintenance activities in accordance with plant procedures. To clean the oxidation from the contacts, the contacts were de-termed and cleaned. Following this maintenance activity, Post Maintenance Testing was completed satisfactorily for the contact, and LCO 3.0.3 was exited at 2211 on March 09, 2015 after successful completion of SR 3.5.1.2. The successful completion of the 72CF Throwover Scheme surveillance test provided reasonable assurance that this contact will continue to perform its design function.
Significant Safety Consequences and Implications:
The purpose of Technical Specification 3.5.1, is to limit the release of radioactive materials to the environment following a Loss of Coolant Accident (LOCA). AC and DC electrical power is required for Engineered Safety Features to function during any analyzed accident with a loss of off site power, neither of which occurred coincident with this event.
Maintaining the required Division 1 and 2 AC and DC electrical power distribution subsystems Operable ensures the RHR LPCI system can automatically supply LPCI water to the reactor core after the reactor pressure has been reduced to less than the RHR System discharge pressure following a LOCA.
Fermi 2 YEAR SEQUENTIAL While the 72CF 480V Swing Bus was inoperable, the automatic transfer required between the two safety-related divisions at the 480V distribution level was lost. The Swing Bus is designed for normal feed from Division 1 Essential Safety Systems (ESS) Bus 72C through two normally closed series contactors, with the standby feed coming from Division 2 ESS Bus 72F through two normally open contactors. These contactors will either close or open automatically as a result of the operational status of the associated breaker.
With the auxiliary contact in a degraded condition likely due to oxidation, the alternate power supply breaker was unable to close, resulting in both RR pump discharge valves and both divisions of RHR LPCI injection valves without their 480V power source. This impacted all RHR LPCI functions. No loss of power to the plant occurred and Divisions I and 2 of Core Spray (CS) and High Pressure Coolant Injection (HPCI) ECCS were available at the time of the event. In addition, the affected valves could have been energized by their normal power source, 72C, at any time during this event by the operating crew, therefore, this event has minor safety significance.
Cause of the Event:
The most likely cause was found to be oxidation which caused the degraded condition of the auxiliary contact.
Corrective Actions:
The contact was cleaned. The successful completion of the operability surveillance demonstrated a reasonable assurance that this contact will continue to perform its design function.
Additional Information:
A. Failed Component: Auxiliary Contact Function: Interlock Manufacturer: ITE-Gould Model Number: 5642-DUGAB Failure Cause: Oxidation of the Contacts Previous Licensee Event Reports (LERs) on Similar Problems:
There are no similar previous events within the past five years.